Alamo Impact Crater Documented

Introduction: The early Late Devonian, wet-target Alamo Event at southern Nevada resulted in one of the best-exposed and well-dated impact deposits ever documented, the Alamo Breccia [1]. Because post-impact geologic processes have modified, obscured [2, 3] and perhaps wiped away its original morphological and architectural relationships, the crater stratigraphy has remained uncertain. Thus, the full Breccia distribution (Figure 1), and the crater location, has been problematic. Carbonate breccias and associated bedrock deformation in Devonian strata exposed on Tempiute Mountain (TM) (Figure 1), exhibit a clear crater strati-graphy. Five main types of deformational units are recognized (Figure 2): (a) parautochthonous target rocks and associated monomict authigenic breccia, (b) allogenic breccia dikes and sills, (c) allogenic fall-back breccia, (d) allogenic, partially melted breccia, and (e) reworked, allogenic breccia. High-pressure, shock-metamorphic effects in minerals and rocks, including PFs and PDFs in quartz grains and conical fractures and related features in disturbed target rock also occur [4]. In addition, the apparently concomitant, ductile and brittle deformation in the upper Lower Devonian Oxyoke Canyon Sandstone (Dox) is linked to the Event. Deformational units at TM: Above the Oxyoke Canyon Sandstone, the Middle to early Late Devonian secuence at TM bears at least five discrete, deforma-tional breccia bodies (Figure 2). These breccias display a variety of grain-sizes, shapes, and stratigraphic relationships. Breccia clasts were derived from both recognized and unrecognized carbonate lithofacies in the area. (1) Allogenic breccia dikes and sills. Unit 1 is comprised of irregular lenses and dikes of variably clast-to matrix-supported, cobble-to pebble-sized, polymict, dolomite breccia. Chaotic clasts are composed of several types of dolomite, quartzite and fossils , and decrease in size to the walls. The matrix (35-70% of total volume) is composed of subequal proportions of monocrystalline shocked quartz grains, and dolomictrite. The dikes pinch and swell and abruptly change direction. Unit 1 significantly differs from any other breccia in the study area and its composition, internal structure, and stratigraphic relationships with adjacent rocks have not been documented at any other Alamo Breccia localities. It is interpreted as a dike-and-sill system injected from a partially lithified sand